In this week’s issue of Science, a study led by researchers at the University of Utah found that,What really limits tree growth is usually not photosynthesis, but cell growth.This suggests that in a changing climate, we need to rethink how we predict forest growth:The forests of the future may not be able to absorb as much carbon from the atmosphere as we thought.
“A tree grows like a wagon moving forward on a road,” said Professor William Anderegg, who led the study. “But we basically don’t know whether the common ‘horse’ is photosynthesis or cell enlargement and division.”
We learn the basics of growing trees at an early age. Trees produce food through photosynthesis, converting sunlight, carbon dioxide and water into leaves and wood. There’s more to this story, though.To convert the carbon obtained from photosynthesis into trees, tree cells need to expand and divide.
Trees get carbon from the atmosphere through photosynthesis, which is what trees docarbon source; trees then use this carbon to build new tree cells, which arecarbon sink.
If the growth of a tree is limited by carbon sources, then it is only limited by how much photosynthesis the tree can do, then the growth of the tree can be predicted relatively easily in a mathematical model. So rising carbon dioxide in the atmosphere should ease that limitation and allow trees to grow taller, right?
But if instead, tree growth is determined by carbon sinks, then trees can only grow as fast as cells divide. Many factors can directly affect photosynthesis and cell growth rates, including temperature, availability of water and nutrients. therefore,If tree growth is determined by carbon sinks, the effects of these factors on carbon sinks must be included when simulating tree growth.
The researchers examined this question by comparing the carbon sources and carbon sinks of trees in North America, Europe, Japan and Australia. Measuring carbon sinks is relatively easy, and researchers collect samples from trees that contain growth records. Measuring the width of each ring on a tree ring basically reconstructs the process of tree growth.
In contrast, measuring carbon sources, while feasible, is more difficult. The source data was measured using 78 vorticity covariance towers at least 9 meters high that measure carbon dioxide concentrations and wind speeds at the top of the forest canopy in three dimensions. Based on these measurements and some other calculations, the researchers can estimate the total forest photosynthesis of the forest stand.
The researchers analyzed the data they collected, trying to find evidence that tree growth and photosynthesis are interrelated or coupled processes. However, they found no such link.When photosynthesis increased or decreased, tree growth did not follow.
“A strong coupling between photosynthesis and tree growth is expected when tree growth is resource-constrained,” said Dr. Antoine Cabon, first author of the paper. prove the conclusionTree growth is not determined by carbon sources. “
Surprisingly,This phenomenon of decoupling and decoupling occurs in a global context.The researchers did expect to see some decoupling in some places, but they didn’t expect this pattern of decoupling to be so widespread.
The researchers are interested in what conditions lead to stronger or weaker decoupling. For example, fruit-bearing and flowering trees and conifers show different carbon-source-carbon-sink relationships.Diversity in the forest increases coupling, while dense, inter-covering leaf canopies reduce coupling.
The study also found that the question of carbon sources and carbon sinks depends on the environment and climate of the trees, meaning that climate change could reshape the map of the world’s forest resources and carbon sink limitations.
An important insight from this study is that vegetation models may need to be updated when mathematical equations and plant characteristics are used to estimate future forest growth, since nearly all current models assume that tree growth is determined by carbon sources.
For example, current vegetation models predict that forests will thrive as carbon dioxide levels in the atmosphere increase. In fact, however, the growth of trees is often restricted, which means thatFor many forests, there may not actually be more forests as carbon dioxide increases.
Forests currently absorb and store a quarter of global carbon dioxide emissions. But if forest growth slows, so does the ability of forests to absorb carbon and mitigate climate change. So this study reminds us that mitigating the climate crisis may be more challenging than we thought.
